This invention relates generally to superhydrophic, self-cleaning, and icephobic polymer coatings, and, more particularly, to durable superhydrophic, self-cleaning, and icephobic polymer coatings.
The surface build up of ice, or ice accretion, and ice adhesion to various surfaces, have been a consistently undesirable occurrence on infrastructure in high altitude and cold regions. The infrastructure affected by ice includes, but is not limited to, wind turbines, power lines, aircrafts, naval vehicles, and buildings. Associated costs with ice accretion and ice adhesion include ice removal, inefficient operation, aerodynamic instabilities, and safety hazards. Current methods of ice removal are characteristically separated by active and passive means. Active methods typically involve the input of mechanical and thermal energy to break or melt ice, as well as the application of sacrificial low surface energy waxes, which all require active involvement of a trained workforce using costly tools and environmentally-unfriendly chemicals. Traditional passive methods include low surface energy coatings, but these surfaces are easily fouled, require maintenance, and tend to be semi-sacrificial. Establishing a surface treatment that required low to no maintenance that reduced the accretion of ice would result in both economical and ecological savings.
Ice accretion, and its adhesion strength, is related to many factors tied to the source of water, as well as the energy of the surface and the droplet. Atmospheric icing, an issue of great importance to aerospace applications, is extremely difficult to resolve. Resulting from their high purity, water droplets in clouds can be found as super-cooled liquid with temperatures reaching as low as −40° C. Upon the high velocity impact to an airplane, the super-cooled water instantly begins ice nucleation, and the surface of the airplane is fouled.
At lower elevations, ground precipitation becomes a problem. Snow, rain, ice pellets, freezing rain, and the combinatory precipitation of the previously listed foul the surfaces of ground infrastructure. Though not as extreme as the high velocity impact of super-cooled liquid with atmospheric icing on planes, ground precipitation becomes more of an issue due to the scale of the problem. Lastly, the formation of ice by condensation of ambient moisture as frost must be addressed. Although the ice layer associated with frost is typically thinner than the accretion developed through the previously listed sources, frost has greater adhesion to the surface due to its deposition throughout the recesses of a surface.
Partially listed above, the factors that affect ice adhesion are numerous and are related to a combination of the environment and the surface characteristics. Beginning with environmental factors, the ambient temperature and humidity have been shown to affect ice adhesion. Impact velocity, droplet size, and wind speed were shown to affect ice adhesion through the droplet kinetics. Relating to both the drop and the environment, the ice nucleation and freezing rate were found to affect ice adhesion. Finally, as mentioned before, the type of icing has serious affects on the strength of ice on a surface.
Moving to the surface characteristics, which are the focus of this work, and the use of superhydrophobic surfaces for anti ice accretion, the hydrophilicity or hydrophobicity of a surface affects ice adhesion. This attraction, related to the surface energy of the surface, typically has been characterized through contact angle measurement, with a contact angle below 90° being hydrophilic, and a contact angle larger than 90° being hydrophobic.
Superhydrophobic surfaces represent one approach, exhibiting a surface's ability to shed water droplets due to a static contact angle greater than 150° and a low contact angle hysteresis. This effect is named after the lotus leaf, which exhibits this behavior due to a low surface energy wax, as well as a hierarchal roughness of nanometer and micrometer asperities. Many research groups have fabricated surfaces similar to the lotus leaf in order to reproduce the water shedding effect, with comparable performance.
In order to fabricate the superhydrophobic surface, a variety of techniques have been used through the literature. These techniques include top-down subtractive methods: optical lithography, e-beam lithography, soft lithography, nanoimprint lithography, block copolymer lithography, scanning probe lithography, and plasma etching, as well as bottom-up additive methods: sol-gel nanofabrication, molecular self-assembly, vapor phase deposition, embedded nanoparticles of silica in epoxy mixture, carbon nanotubes in thermoplastic, cast silica/POSS in fluoroalkylsilane, and silanized calcium carbonate in polyacrylate. No matter which techniques or base substrates were employed, each method included micro/nanometer features for roughness and a low surface energy coating.
Although many research groups have worked on the creation of superhydrophobic surfaces, the focus of their use against ice accretion has been studied by just a few. Tourkine and his group, compared the effect of superhydrophobicity on the freezing of static water droplets compared to hydrophilic surfaces By comparing a fluorinated thiol treated microstructured copper substrate versus a smooth copper substrate, a delay in freezing time was observed when using a superhydrophobic surface. The argued reasoning for this freeze delay was the presence of an air film between the droplet and the superhydrophobic surface, providing insulation to the droplet. Coupled with the self-cleaning properties of superhydrophobic surfaces, the probability increases for the water droplet to be shed prior to adhering to the surface. In 2010, Yin et al. showed surface wetting for surfaces, from superhydrophilic to superhydrophobic, were temperature-dependent. As the ambient testing temperature was reduced from 40° to −10° C., greater wettability change was shown on the superhydrophobic surface on a horizontal surface. Although there was greater change associated with superhydrophobic surfaces, the paper also looked at varying inclination angle during ice accretion. Because the superhydrophobic surface was able to shed the super-cooled water spray, the amount of ice accretion was significantly decreased with increased inclination angle and hydrophobicity. In Antonini et al., a superhydrophobic etched aluminum surface coated with Teflon was found to reduce the amount of run-back ice as compared to the untreated aluminum surface and a hydrophobic PMMA coated aluminum surface, designed to replicate the airfoil of an airplane wing. Although the superhydrophobic surface did develop an ice layer, it showed a reduction in energy required to remove ice accretion. The reasoning for the reduced run-back ice was again linked to the ability of the surface to shed water droplets, as well as a reduced wetting trail of the droplet on the surface during roll-off. Alizadeh et al. discovered a delay in ice nucleation when comparing superhydrophobic surfaces versus superhydrophilic, hydrophilic, and hydrophobic surfaces. Using a dynamic droplet, and high-speed photography, superhydrophobic surfaces were found to have dual means of increasing the time of ice nucleation—they affirmed the reduced heat transfer by means of an air barrier below the droplet that Tourkine noted, as well as suggesting a reduced nucleation initiation between the droplet and the surface due to an increase in nucleation activation energy to form a nucleating site. Additionally, the superhydrophobic surface provided a more elastic response of the droplet upon impact as compared to the other surfaces.
Mishchenko et al. showed that the adhesion strength of ice on the surface of a superhydrophobic surface resulting for static freezing was much less than that of the compared surfaces. Further, the retraction of the droplet upon impact of a superhydrophobic surface was shown to provide an additional means of removal prior to ice nucleation. In the study a variety of superhydrophobic surfaces were made using nanostructured silicon arrays, treated with a hydrophobic silane. The nanostructures were selected to see the dynamic pressure stability of the different structures and their effect on impacted droplet retraction. Closed cell structures were found to have better pressure stability, and a reduction in energy loss during droplet retraction. Kulinich and coworkers showed in 2009 that the strength of ice adhesion was less linked to the static contact angle, and was more of a function of the contact angle hysteresis. By using a centrifugal ice shear strength test, the ice adhesion was measured to see its comparison to both contact angle and contact angle hysteresis.
While much has been reported on the use of superhydrophobic surfaces for anti ice accretion, several research groups suggest against the use of superhydrophobic surfaces due to their inherent roughness; an essential characteristic of superhydrophobic surfaces since the highest contact angle that would result from a smooth surface is on the order of 1200.
Previously mentioned is the ability for superhydrophobic surfaces to support water droplets from impinging the surface, typically described as being in the Cassie-Baxter state; however, with sufficient energy, the water droplet may be impaled by the surface asperities. This state is typically described as the Wenzel state. Once the droplet is impaled onto a surface, either by the kinetics of the droplet or the asperity pitch of the surface features increasing too much to support the Laplace pressure of the static droplet, the ice formed would again be strengthened by the increase in contacted surface area.
A further issue related to the features of superhydrophobic roughness is the cycling of icing and deicing. Kulinich showed that the repeated process of a passive anti-icing surface lost its effectiveness after each cycle, questioning the durability of these features. Due to the high aspect ratio of the features, the brittle cleaving of the features changes the characteristics of the surface.
Although there exists several hurdles with the use of superhydrophobic surfaces for use in passive anti ice accretion applications, including but not limited to: cyclic fracture of surface features and the increased adhesion due to contacted surface area from the transition between the Cassie-Baxter and Wenzel states and frost formation, the use of superhydrophobic surfaces should not be ruled out as a possible solution to anti icing.
The route to obtaining the superhydrophobicity is through the combination of the surface geometry and chemical functionalization of the silane. Prior work has demonstrated the potential for preparation of superhydrophic surfaces from combinations of silica, silanes, and POSS′. Rios et al (Transparent Ultra-hydrophobic Coating) showed a contact and sliding angle of above 165° and less than 1° could be achieved with two coating solutions. The coatings were composed of hydrophilic fumed silica (Aerosil 200) and fluoro-functionalized polyhedral oligomeric silsesquioxane (FPOSS). The concentration ratios of Aerosil 200 and FPOSS in the two solutions were 1% wt. to 3% wt. and 0.5% wt. to 1.5% wt., respectively. Rios et al. previously tested multiple ultra-hydrophobic coatings for durability under multiple conditions. Many aspects of durability were tested including QUV stability, water immersion, isopropyl alcohol (IPA) immersion, and paper rubbing. One of the best performing coatings was the UH2 comprised of hydrophobic dimethyl-silicone treated fumed silica (Cab-O-Sil TS720) and Dynasylan F 8263. The coatings, while demonstrating good icephobic characteristics, were not durable and were easily removed. Thus, there exists a need for a durable and low maintenance coating providing icephobic characteristics.